1. Field of the Invention
This invention relates to apparatus for hydraulic control of a subsea device, and more particularly to hydraulic apparatus for the individual control of a relatively large number of subsea well devices using only a few hydraulic pressure source lines from a surface vessel to the seafloor.
2. Description of the Prior Art
The production of oil and gas from offshore wells has developed into a major endeavor of the petroleum industry. Wells are commonly drilled several hundred or even several thousand feet below the surface of the ocean, substantially beyond the depth at which divers can work efficiently. As a result, the drilling and operating of a subsea well must be controlled from a surface vessel or from an offshore platform. The testing, production and shutting down of the subsea well is regulated by a subsea Christmas tree which is positioned on top of the subsea wellhead. The Christmas tree includes a plurality of valves having operators which are biased to a non-active position by spring returns, and it has been found convenient to actuate these operators by hydraulic fluid which is directly controlled from the surface vessel. For this purpose, a plurality of hydraulic lines are commonly run from the surface vessel to the wellhead to open and close these valves, and to actuate other devices in the well and the wellhead during installation, testing, and operating the subsea well equipment, and also during workover procedures being performed on the well.
In some of the prior art systems a separate hydraulic line is run from the surface vessel to each of the hydraulically powered devices at the seafloor. Some of these hydraulic lines may be run through a riser, but for many of the subsea operations the riser is too small to contain all of the lines required. A common solution is to employ additional hydraulic lines that are stored on a reel located on the surface vessel, which are made up into a hose bundle that is connected to the outside of the drill pipe or riser and lowered therewith to the seafloor. However, such a hose bundle is expensive, and is heavy and cumbersome to handle simultaneously with the drill pipe or riser, particularly in deep water.
Other prior art equipment uses an electrical cable that is fed off a reel located on the surface vessel as the riser is lowered to the well in a manner similar to the hose bundle. This cable is also expensive, heavy and cumbersome to handle when used outside the drill pipe or riser. Another disadvantage of using an electrical cable inside the drill pipe or riser is that the cable must be in sections, and these sections must be connected together in an end-to-end arrangement at the junction of each section of pipe or riser. This means that a very large number of connections must be made when numerous pipe or riser sections are involved, and each of these connections must function properly in order for the system to work. It has proved to be quite a difficult problem keeping all of these electrical connections working properly in a subsea environment.
What is needed is apparatus which can be used to control a large number of subsea operators with only a few hydraulic source lines between the surface vessel and the wellhead. In some systems this small number of lines could be contained inside the riser or drill pipe. In other systems some of the hydraulic lines could be inside the riser or drill pipe and a few additional lines could be contained in the hose bundle. In either case, a reduction in the number of hydraulic source lines would reduce the expense and the difficulty of handling the hose bundle.
One prior art device that is used in a system for controlling a plurality of remotely positioned hydraulically actuated underwater devices by a single hydraulic control line is disclosed in U.S. Pat. No. 3,993,100, issued November 1976 to Pollard et al. The Pollard et al device involves a plurality of valves each having a pilot, and with the pilot of each valve arranged for actuation by a different pressure level in a signal manifold that is connected to all the pilots.
Another prior art apparatus for this purpose is disclosed in U.S. Pat. No. 3,952,763, issued April 1976 to Baugh. This apparatus includes a valve having a single inlet port and a plurality of outlet ports arranged so that the outlet port that is connected to the inlet port is determined by the magnitude of the pressure that is applied to said inlet port.